This invention is a silicon-based monolithic integrated circuit which is capable of operating at microwave frequencies from the megahertz to the gigahertz range.
Until about ten years ago, solid state microwave circuits were fabricated exclusively from discrete components that included active semiconductor devices such as transistors and diodes. Even today, the field is shared between this older approach, which to a large extent prevails as the low cost alternative, and the newer monolithic integrated circuit devices. Whereas discrete components are often made using silicon bipolar technologies, monolithic microwave integrated circuits have so far been made primarily in gallium arsenide (GaAs). Monolithic microwave integrated circuits offer improved bandwidth over those that are fabricated from discrete components. This is because the integrated circuit implementation allows the placing of matching networks sufficiently close to individual transistors to avoid electrical losses and to eliminate deleterious parasitic capacitances due, for example, to wire bonds. There is a concomitant reliability advantage that becomes extremely beneficial in applications requiring large numbers of elements or devices, typical of airborne phased array radar implementations, for example. In this and similar applications, each module of the array system may typically require up to three chips incorporating power amplifiers, low-noise amplifiers, and phase shifters. The benefits of integration for microwave applications until now have been available exclusively from gallium arsenide devices.
One reason that GaAs has been chosen for basic electronic functions is that this material has the high value of electron mobility that uniquely enhances device performance particularly at higher frequencies. While discrete silicon bipolar transistors can be and are used at microwave frequencies, integrated silicon implementations which have lower mobility are generally inferior at the higher microwave frequencies. Electron mobility is not the only physical parameter that is in favor of GaAs. The highly insulating quality of the GaAs material also favors high frequency performance relative to silicon. Ordinarily, silicon starting material is several orders of magnitude more electrically conducting than GaAs and this property ultimately limits the maximum available gain that can be delivered at high frequencies by devices fabricated in silicon. It is now believed that it is as much the insulating quality of the GaAs substrate that preferentially enhances the latter's performance relative to silicon in the lower gigahertz ranges as it is the mobility advantage. This insulating quality inhibits parasitic current paths between transistor electrodes on the same chip that would otherwise adversely affect its performance as a microwave integrated circuit.
Historically, and in spite of the fundamental electronic advantages of GaAs as described above, the utilization of this material on a large scale has been slow due to problems in manufacturing. These problems have included the unavailability of consistently high quality GaAs substrate material, processing methods not oriented toward the high quality substrate material, processing methods not oriented toward the high wafer start rates that in silicon have tended to support the evolution of a competitive manufacturing technology, and the basic problems associated with a compound semiconductor as opposed to an elemental one. These are generally reflected in the difficulty of process control and yield maintenance which adversely affect the unit cost. Added to this is the physical as well as the chemical fragility of the material which further complicates manufacturing even from the boule-growth stage. Gallium arsenide wafers are notorious for this brittleness which typically results in only half the started wafers surviving to the RF test point. Whereas the silicon industry is presently gearing up to process 200 mm wafers, GaAs wafers are generally available only up to 75 mm diameter, and at a significantly higher cost than silicon. Since GaAs production today is a small- fraction of the market for silicon-based devices, it is the latter that drives the important wafer processing equipment infrastructure. Thus the cost penalty associated with the superior electronic performance of GaAs is a severe one.
A subset of silicon CMOS technology is so-called SOI (Silicon-On-Insulator). During the last decade, SOI implementations have become highly favored for making radiation-hard signal processing integrated circuits. A subset of SOI is SOS (Silicon-On-Sapphire). This technology addresses radiation hardening requirements by improving the electrical isolation of the components on the substrate. In particular, distributions of excess electrons created by bombarding radiation are effectively confined and prevented from causing electrical upsets or "soft" errors. The same electrical isolation technique also offers a higher frequency advantage. However, the problem with SOS material is that there is an electronically imperfect interface between the insulating sapphire material on which the active silicon is deposited and the active silicon itself. This results in a back channel leakage effect. Whereas the imperfections giving rise to this effect do not impede radiation hardness per se, they tend to be deleterious with respect to normal device performance and to impact adversely the yield of circuits that can perform to full operating specifications. Back channel leakage would be particularly disadvantageous at microwave frequencies because it severely limits the maximum usable gain. Added to these limitations is the minimum thickness of device grade silicon that can be isolated above the non-conducting sapphire substrate. Now, however, there are alternatives to silicon on sapphire.
In the last several years, a new SOI radiation hard silicon materials technology has emerged. This is called Separation by the IMplantation of OXygen (SIMOX). To make a SIMOX wafer, one implants a large dose of oxygen into the subsurface region of the wafer. Annealing then converts this oxygen-rich region to a subsurface insulating glass film. The silicon surface region through which the implantation is made simultaneously reverts to active, device-quality silicon in which circuitry can be synthesized in much the same way as it is in SOS. The advantage of this technique over SOS is that the thickness of the active silicon layer can be made substantially less than it is in SOS, thus further confining the effects of ionizing radiation. Furthermore, the back channel leakage effect is also minimized. However, while the active devices and their various electrodes are indeed decoupled from the substrate in a direct current sense, they remain capacitively coupled to the conducting substrate and therefore to each other at microwave frequencies because of the conducting properties of the silicon starting material. In other words, in spite of their subsurface insulating film, even SIMOX-based devices are not suited to deliver microwave performance because the unimplanted silicon below the implanted insulating layer is conducting at microwave frequencies.